This could have a lot of meanings... could mean to separate in pieces or destroy something, to break an idea and make it easy to digest or understandable for someone else, if your car suddenly stopped working, it BROKE DOWN ... etc, etc, etc. A little bit of context would be really helpful ^^

Let's break it down. break down = To cause to collapse, destroy. e.g. let's break the wall down. It means that we want to take the wall down. To have a collapse, that something is ruined.

I think you allow us to make the context for you! This is even more fun! You are on your routine work with a friend facing a (seemingly) difficult Physics problem, while preparing a successful expedition to Jupiter; you need to define the temporary sequence of hydrazine engines thrusts for a quiet landing on those yet untouched landscapes; you are more confident than your friend who is only a freshman at Las Vegas City college (nobody knows --- not even Nasa --- how he got a job for Nasa), so you say: a) 'let's break it down', or b) 'let's break this computation down', meaning: we try and we'll surely solve it, although we might have to struggle a little bit. Within a few weeks, when everything seems ready, will you dare to step on that spaceship ?

explain. as in ' thanks for breaking it down for me'